Conventionally, a method has been known, which performs positioning of a movable body by using GPS satellites (GNSS satellites) and then receives a correcting signal from a known reference station and corrects the positioning result to obtain a position of the movable body with higher accuracy. This method is generally called a differential GPS (differential GNSS).
In Japan, the differential GPS (hereinafter, may be abbreviated to DGPS) for ships has been operated by Japan Coast Guard. Specifically, a number of DGPS reference stations have been installed domestically to cover coastal sea areas of Japan, and each reference station performs positioning by receiving radio waves from GPS satellites. Since the latitude and the longitude of each reference station is known in advance, a GPS positioning error is calculated based on the positioning result, and the error is broadcasted from the DGPS reference station as correcting information (DGPS beacon). A navigation apparatus equipped in a ship and including a DGPS beacon receiver receives the result of the positioning performed by the GPS and corrects the result based on the correcting information received by the DGPS beacon receiver.
Such a DGPS beacon transmits data according to RTCM-104 which is a standard defined by Radio Technical Commission for Maritime Service. A format called type 16 for transmitting a special message is set for RTCM-104. Currently in Japan, a service in which meteorological information and hydrographic information observed at predetermined observed locations (e.g., beacons) close to DGPS reference stations are transmitted as a type 16 message of the DGPS beacon is in operation (meteorological and hydrographic report). Thus, information regarding meteorological and hydrographic phenomena which is important in traveling can be provided to an operator.
In this regard, JP2004-258020A discloses a configuration in which meteorological and hydrographic information is acquired by analyzing the special message written in type 16 of the DGPS correcting information, and the meteorological and hydrographic information from the respective observed locations and a location of the ship are displayed on an electronic chart display and information system in the form of wind arrows and the like. Thus, in JP2004-258020A, the relation of the meteorological and hydrographic information from each observed location with the ship becomes obvious, and therefore, the meteorological and hydrographic information from each observed location can be grasped comprehensively.
Moreover, JP1993-240963A discloses a marine meteorological change situation display device which, although does not acquire meteorological and hydrographic information broadcasted by the DGPS beacon, acquires a weather map by facsimile and displays a track of a ship or the like by superimposing on the weather map.
With the configuration in JP2004-258020A, there have been cases where, when the meteorological and hydrographic information is acquired from a number of observed locations, information displayed on a nautical chart becomes messy and it is difficult to efficiently obtain the information truly desired by an operator. Moreover, also in JP1993-240963A, since the location of the ship is displayed in the weather map, there have been cases where confusion is caused because of the excessive amount of information. Therefore, it has been desired to improve such conventional configurations in view of tidily displaying the meteorological and hydrographic information.